Null in my contact

No impact on privacy, as far as I know. Simple press "Edit" in the top right corner, then select the label for the number, which will be just to the left of it. You can then change it to whatever you like.

The value 'null' is used whenever the device cannot find an expected value. In this case, it most likely means that it cannot retrieve the type that the number corresponds to. In the example below (from Apple), it is distinguished as a 'mobile' number.

Tap on "Edit" in the top right corner of the screen and change the label for that phone number to anything you wish. I would label it as "mobile" or "home" or "work" because there are no FaceTime icons to indicate that this person uses an iPhone. Good luck.

There is no apparent impact on privacy, as Bubba said. My best guess as to what happened is that somehow your "mobile" label in Contacts got corrupted or accidentally erased so, after that, every reference to that label failed to resolve.